The invention relates to hydraulic accumulators and more specifically to methods of venting hydraulic accumulators.
It is known to utilize hydraulic accumulators in conjunction with hydraulic brake systems equipped with an antilock system or a traction control system. The hydraulic accumulators temporarily receive excess brake fluid from wheel-mounted brake cylinders in order to reduce the pressure in the brake cylinders in a sufficiently rapid manner. The brake fluid is later withdrawn from the accumulator as needed.
Accumulators of this type may be sealed from the environment, trapping a volume of air inside the spring chamber or behind the accumulator piston. It is often advantageous to vent the accumulator so that trapped air does not affect the performance. Venting the trapped air to a larger air volume relieves the high air pressures in the accumulator that could adversely affect the function of the accumulator.
U.S. Pat. No. 4,769,990 to Bach et al. illustrates a combination accumulator and variable volume sump for use in a vehicle antilock brake system. The accumulator includes a piston with a reduced diameter section in communication with a vent, which in turn communicates with the atmosphere. Air in the system is displaced via the vent.
Vented accumulators present several practical problems. Vents are typically formed by drilling through the housing and directly into the accumulator bore. The drilling process leaves raised burrs on the inside wall of the accumulator that snag on and damage the piston and/or its sealing member (i.e., an O-ring) as it is inserted into the accumulator bore during assembly. This damage is difficult, if not impossible, to detect in a sealed accumulator.
Furthermore, the raised burr may hinder the piston stroke during operation of the accumulator, causing further damage to the piston and/or its sealing member, as well as potentially impairing the operation of the hydraulic braking system. To remedy any hindrance created during operational movement of the piston, the piston must often be designed to include a reduced diameter portion over the range where direct contact would otherwise be experienced. This may require extra machining operations and/or difficult injection molding techniques.
Attempts to eliminate the inevitably produced burrs have resulted in intricate and time consuming de-burring processes. The de-burring processes are complicated by the size and shape constraints of the accumulator bore. Finding the proper equipment to fit inside the accumulator bore, in addition to finding an operator who can adequately remove the burr from the cylindrically toleranced wall without leaving edges or ridges that will pose the same problems as the burr itself, has proven difficult.
Another practical problem with drilling the vents directly into the accumulator bore arises in light of the need to minimize the cross-sectional area of intersection between the accumulator wall and the vent hole. It is often necessary, due to the configuration of the housing and various other components, to drill at least one common passageway that is shared by distinct vent bores, thereby connecting the common passageway to individual accumulators. The distinct vent bores must be drilled to intersect the common passageway and then intersect the accumulator bore perpendicularly. This often requires multiple drilling operations as well as the need to close off the distinct vent bores after the connections have been made. This technique is time-consuming and burdensome.
The present invention alleviates the above-identified problems with venting hydraulic accumulators. The invention provides an accumulator having a vent that poses virtually no risk of damage to the piston or the piston sealing member and presents virtually no obstacles to the successful operation of the accumulator and the braking system as a whole. The vent is fast and easy to construct with minimal operational steps and no complex tooling or techniques.
Specifically, the invention provides a hydraulic accumulator assembly including a pump housing having therein at least one accumulator bore. The accumulator bore has an axis and an inner wall. A piston is movable within the accumulator bore and divides the accumulator bore into an accumulator chamber, which is communicable with the hydraulic brake system, and a spring chamber. A spring in the spring chamber biases the piston in the direction of the accumulator chamber. A passageway in the pump housing is closely spaced from the inner wall so as to form a relatively thin wall thickness between the passageway and the inner wall. A vent bore is punched in the inner wall and communicates between the spring chamber and the passageway. The vent bore is punched in a direction away from the axis.
In one embodiment, the relatively thin wall thickness between the passageway and the inner wall is approximately 0.5 mm. In another embodiment, the vent bore is formed by a punching stroke substantially perpendicular to the axis.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings.